The blood pressure of rabbits with chronic one-kidney hypertension was lowered to normal by direct immunization with a preparation from hog kidney cortex that did not contain renin. It was concluded that an unknown substance elicited an antibody in the hypertensive rabbits that cross-reacted with a factor essential for the maintainance of an elevated blood pressure level. As a consequence of these experiments, a new pressor substance, named renopressin, has been discovered in extracts of rabbit kidney. Subcutaneous injections of renopressin into normal rabbits causes, after 48 hours or more, a gradual and moderate increase in blood pressure. Once an elevated level has been attained it may be sustained for at least 94 days although the administration of the extract was stopped after 9 days. Preparations containing rabbit renopressin are not pressor in hypertensive rabbits whose pressure have been lowered to normal by direct immunization with hog kidney cortex preparations. Thus, the antibody that neutralizes the causative agent responsible for the hypertension also neutralizes the renopressin that is injected and implies that these two substances may be similar if not identical. BIBLIOGRAPHIC REFERENCES: Chronic One-Kidney Hypertension in Rabbits I. Treatment with Kidney Extracts, By Leonard T. Skeggs, Joseph R. Kahn, Melvin Levine, Frederic E. Dorer, and Kenneth E. Lentz, Circ, Res., Vol. 37 715-724, 1976. Chronic One-Kidney Hypertension in Rabbits II. Evidence for a New Factor., By Leonard T. Skeggs, Joseph R. Kahn, Melvin Levine, Frederic E. Dorer, and Kenneth E. Lentz (Circ. Res., In Press, 1976).